Yesterday I bought a map stamp for the map CP_Steel, but when I visited that map I didn't see any special hat effect.
I am not sure the effect is visible if you stand still or if you taunt. You have probably to move to see the particle effect of World's Traveler hat.
Try to play on CP_Steel on a server with replay available, save one replay and verify if the particle for your hat is visible. As soon as I arrive home I will try and confirm that.